Primary production of hydrocarbons from a subterranean hydrocarbon bearing formation usually occurs by allowing the energy of the formation to drive the hydrocarbons from the formation to the surface through production wells. The field energy may be from water drive, gas cap or solution gas drive or various combinations to create a natural flow of the hydrocarbons upwardly through the production well stream. In the gas drive reservoir, gas provides the major source of energy to drive the hydrocarbons toward and up the production well. If there is no initial gas cap, the producing mechanism is a solution gas drive and as long the pressure within the formation is sufficiently high, gas remains in the solution with crude oil. As production is continued by solution or gas drive, pressure decreases to a level in which gas begins to come out of solution and forms a continuous free gas phase. The free gas then flows through the formation with little or no displacement of hydrocarbon, thereby losing the drive mechanism for the reservoir.
Secondary recovery operations typically commence when the pressure of the formation has decreased to a level indicative that energy within the formation has been substantially expended. Water flooding has often been used as a secondary recovery method and includes the injection of water into the formation through one or more injection wells. The water is forced toward the production wells, thereby resulting in washing or displacing the hydrocarbons toward the production wells and resulting in additional recovery. The water injection must necessarily be conducted under pressure greater than the pressure of the formation and depends upon various reservoir factors.
Additionally, gas injection is known as a method of secondary recovery in which gas such as air, flue as or the like is injected into the formation to attempt repressurization and recommencement of the gas drive mechanism.